1. Field of the Invention
The present invention generally relates to apparatus for measuring the flow velocity of fluid using an ultrasonic beam. More particularly, it relates to ultrasonic flow velocity measuring apparatus which generates or receives ultrasonic beam whose frequency is modulated according to PN (Pseudo Noise) code of a diffusion band of a transit side, and precisely measures a flow velocity of a pipe conduit or sluice open channel.
2. Description of the Prior Art
Conventionally, it is well known to use an ultrasonic flowmeter using ultrasonic beam in order to measure a flow quantity a large-sized pipe conduit or a larger river.
The conventional ultrasonic flowmeter widely uses a flow velocity measuring method using a ultrasonic transit time difference. FIG. 1 illustrates an example that ultrasonic transducers 1 and 2 are installed to be separately from each other in a conventional ultrasonic flowmeter. The ultrasonic transducers 1 and 2 alternately generate or receive ultrasonic beam, and measures a flow velocity by using the following equation (1).
V=xcex94txc2x7C2/2xc2x7Lxc2x7cos xcfx86=(L2/2d)xc2x7[(t21xe2x88x92t12)/(t12xc2x7t21)]xe2x80x83xe2x80x83[Eq. 1]
Herein, xcex94t is equal to t12 and t21 are times that ultrasonic beam is transmitted in fluid at an angle xcfx86 or on the contrary to a flow velocity direction. L is an interval between tow ultrasonic transducers, d is equal to L cos xcfx86 and C is a sound velocity in fluid (called instead of an ultrasonic transit velocity below).
The flow velocity measuring method using the ultrasonic transit time difference previously inputs a predetermined constant L2/2d, and computes a time difference between a time t12 and a time t21, wherein the time t12 is measured when the ultrasonic beam is emitted in a flow velocity direction, and the time t21 is measured when the ultrasonic beam is emitted in opposite direction of the flow velocity direction of the time t12. Such a flow velocity measuring method is well known to those skilled in the art by U.S. Pat. No. 5,531,124(Jul. 2, 1996) and Japanese Patent No. 2676321(Jul. 26, 1998).
However, according to the aforementioned prior method, if an interval L between the ultrasonic transducers is relatively longer, or various sizes of vortexes or eddies occur in the fluid flow, or the suspension concentration of fluid and a temperature distribution change in a natural river, a sound pressure of an ultrasonic beam is severely pulsated at an ultrasonic receiving place because the ultrasonic beam is refracted or diffused, or the absorbing damping factor is changed.
Furthermore, even if an ideal ultrasonic beam having a short wave length is transmitted, the receiving signal becomes a bell-shaped pulse, because the higher harmonic component of the ultrasonic beam is severely damped. For it, a receiving error corresponding to a few periods of the ultrasonic beam usually happens in checking out the moment that the ultrasonic beam is received, and the receiving failure case is not quite less.
In order not to distort the shape of the received pulse in transmitting and receiving the ultrasonic beam, a wideband amplifier is used, but various noises are amplified. Especially, it causes the confusion in measuring the ultrasonic transmitting time due to the pulse noises.
In considering the above problems, another prior art which measures an ultrasonic transit time by emitting or receiving a frequency-modulated ultrasonic beam and obtains a flow velocity is disclosed in U.S. Pat. No. 6,012,338 which is shown in FIG. 2.
As shown in FIG. 2, a frequency modulation oscillator 3 is connected to a transducer switching part 14 through an output amplifier 6. A pair of ultrasonic transducers 1 and 2 are connected to the transducer switching part 14. The ultrasonic modulation oscillator (3) connected to an input terminal of the output amplifier 6 successively outputs an oscillation frequency f when there is no pulse input from an one-shot multivibrator 4, and outputs a frequency fo (shown in FIG. 3c) which is modulated according to a short pulse (shown in FIG. 3b) generated from the one-shot multivibrator 4 by a long pulse (shown in FIG. 3a) generated from a control square pulse oscillator 5 with a given period.
The frequencies f and fo generated from the frequency modulation oscillator 3 pass through the output amplifier 6, and are input to the transducer switching part 14. The transducer switching part 14 inputs the amplified frequencies f and fo into the ultrasonic transmitting transducer 1. The ultrasonic transmitting transducer 1 successively emits the oscillation frequency f and a frequency-modulated frequency fo as shown in FIG. 3d. The ultrasonic receiving transducer 2 installed to a lower place of the ultrasonic transmitting transducer 1 receives the oscillation frequency f and the frequency-modulated frequency fo.
At this time, the output signal of the output amplifier 6 is transmitted to a frequency discriminator 9 through an attenuator 13 and the output switching part 8. The frequency discriminator 9 generates an output voltage (shown in FIG. 3e) during a duration time of the frequency fo. The output voltage of FIG. 3e is changed to a square pulse by a pulse shaping part 10, as shown in FIG. 3f. A time interval measuring part 11 starts a counting operation from a moment at which the square pulse is received. After that, at the moment of a pulse trailing edge, an output switching part 8 and the transducer switching part 14 are switched according to a control of the control square pulse oscillator 5, an output signal (shown in FIG. 3g) of the receiving amplifier 7 is input to the frequency discriminator 9, and an output voltage (shown in FIG. 3h) are changed to a square pulse (shown in FIG. 3i) by the pulse shaping part 10 and is then transmitted to the time interval measuring part 11. At this time, the time interval measuring part 11 stops a counting operation. In addition, the time interval measuring apparatus outputs the counted ultrasonic transit time t12 to a flow velocity arithmetic-logic processing unit 12.
After that, the transducer switching part 14 transmits an output signal of the output amplifier 6 to the ultrasonic receiving transducer 2 by a control of the control square pulse oscillator 5, and emits the ultrasonic beam having a modulated frequency to the ultrasonic transmitting transducer 1. By the aforementioned operation steps, a ultrasonic transit time t21 are measured. The flow velocity arithmetic-logic processing unit 12 receives a time t21 having an opposite direction of the time t21 from the time interval measuring part 11, and calculates a flow velocity by using the above equation (1).
The aforementioned prior art of U.S. Pat. No. 6,012,338 measures an ultrasonic transit time by catching a moment at which a frequency of a receiving signal is changed, and thus measures it even in a condition of a sound pressure of ultrasonic beam is pulsated.
However, the ultrasonic beam generated from the ultrasonic transmitting transducer 1 is reflected from a surface or a bottom, and is transmitted to the ultrasonic receiving transducer 2 after a delay operation, so that it is difficult to capture accurate frequency modulation time point. In other words, as shown in FIG. 4, when the ultrasonic transmitting transducer 1 emits an ultrasonic beam to the ultrasonic receiving transducer 2, the ultrasonic beam is transmitted to the ultrasonic receiving transducer 2 via a multiple-path. For example, the ultrasonic beam through first to third path P1, P2 and P3 has a predetermined phase difference (shown in FIGS. 5a-5c) according to a path difference. At this time, as shown in FIG. 5d, there are many output voltages Vo1, Vo2 and Vo3 in the frequency discriminator 9. Due to Vo1, Vo2 and Vo3, at a receiving side, the moment at which an oscillation frequency f is changed to a frequency-modulated frequency fo is not adequately detected, thereby causing a measurement error.
For example, under the condition that a time interval L between two ultrasonic transducers 1 and 2 is very short (e.g., 0.05 m), a flow velocity V is very slow (e.g., 0.1 m/s), and a sound velocity C is about 1500 m/s, if the measurement error is excluded, the ultrasonic transit time at is to be a value of 3.14xc2x710xe2x88x929 s. When the ultrasonic transit time xcex94t is measured within allowable error range of 1% at a high precision, absolute error of the measured transit time differences should not exceed a value of 3xc2x710xe2x88x9211 s. For this condition relating the absolute error, the transit time measuring apparatus becomes complicated, apparatus for capturing a receiving moment of ultrasonic modulated pulse should be a very stable and precise system, thereby increasing a cost of a manufactured product.
In addition, the flow velocity in a curved passage occurs a large flow velocity deviation at a different measuring place, so that a plurality of ultrasonic transducers should be installed to measure an ultrasonic transit time difference. But, if only two transducers are used like the above prior art, the measurement error of a flow velocity becomes wider, so that a precise measurement operation is not achieved.
The present invention has been made in an effort to solve the above problems. It is a first object of the present invention to provide an ultrasonic flow velocity measuring apparatus which measures a transit time under the condition a synchronization of an ultrasonic signal having a modulated frequency by PN (pseudo noise) code of a diffusion band at a transmitting side is locked at a receiving side, and prevents an excessive measurement error exceeding allowable error range.
It is a second object of the present invention to provide an ultrasonic flow velocity measuring apparatus, wherein a transmitting side and a receiving side use the same PN code, and thus the receiving side receives its own ultrasonic modulation signal even if several pairs of ultrasonic transducers are used.
It is a third object of the present invention to provide an ultrasonic flow velocity measuring apparatus which installs a plurality of ultrasonic transducers to every position, and accurately measures average flow velocity in a curved passage.
To achieve the first object, in an ultrasonic flow velocity measuring apparatus which installs a pair of transducers at a upper stream and a lower stream of a fluid passage and measures a flow velocity according to an ultrasonic transit time difference between the ultrasonic transducers, an ultrasonic flow velocity measuring apparatus includes: a transmitting part for generating an ultrasonic signal having a modulated frequency according to a PN code of a diffusion band; a transducer switching part for alternately applying the ultrasonic signal from the transmitting part to a pair of ultrasonic transducers, and switching a connection state of the pair of ultrasonic transducers for a transmitting or receiving operation; a receiving part for demodulating an output signal of the transducer switching part, and capturing a moment at which its own signal is identical with a signal of the transmitting part; a signal synchronization part for locking a synchronization of a receiving signal when the receiving part captures a signal; a time interval measuring part for measuring an ultrasonic transit time under the condition that a synchronization of a receiving signal is locked in the signal synchronization part; and a controller for controlling a switching operation of the transducer switching part, and calculating a flow velocity according to an ultrasonic transit time in opposition to a flow velocity measured by the time interval measuring part.
The transmitting part is comprised of a transmitting PN code generator, a frequency modulator, and an output amplifier. The transmitting PN code generator includes an oscillator of generating a clock signal of a predetermined period therein, and generates PN code which is determined by a high-level pulse width and a low-level pulse width according to a clock signal generated by the oscillator.
The receiving part is comprised of a receiving amplifier, a frequency demodulator, and a signal capturing part. The signal capturing part detects a moment at which a demodulated signal series is identical with a PN code signal series generated from the transmitting PN code generator.
The signal synchronization part is comprised of first and second synthesizers, first and second LPFs (low pass filters), a differential amplifier, a loop filter, a VCO (voltage controlled oscillator), and a receiving PN code generator. The receiving PN code generator generates the same PN code as an output signal series of the transmitting PN code generator according to an input frequency of the VCO oscillator, after receiving an enable signal from the signal capturing part. Then, the receiving PN code generator generates a PN code of which phase is leading by a predetermined period of the first synthesizer, and generates a PN code of which phase is lagged by a predetermined period of the second synthesizer.
According to a second object, in an ultrasonic flow velocity measuring apparatus which installs a pair of transducers at a upper stream and a lower stream of a fluid passage and measures a flow velocity according to an ultrasonic transit time difference between the ultrasonic transducers, an ultrasonic flow velocity measuring apparatus includes: a transmitting part for selecting one PN code among a plurality of PN code signal series according to a signal selection signal, and generating an ultrasonic signal having a modulated frequency according to the selected PN code; a transducer switching part for alternately applying the ultrasonic signal from the transmitting part to a pair of ultrasonic transducers, and switching a connection state of the pair of ultrasonic transducers for a transmitting or receiving operation; a receiving part for demodulating an output signal of the transducer switching part, and capturing a moment at which its own signal is identical with a signal of the transmitting part; a signal synchronization part for locking a synchronization of a receiving signal when the receiving part captures a signal; a time interval measuring part for measuring an ultrasonic transit time under the condition that a synchronization of a receiving signal is locked in the signal synchronization part; and a controller which generates a signal selection signal for selecting one PN code among a plurality of PN code signal series at the transmitting part, and calculates a flow velocity according to an ultrasonic transit time in opposition to a flow velocity measured by the time interval measuring part.
According to a third object, an ultrasonic flow velocity measuring apparatus includes: a plurality of set units which emit or receive an ultrasonic beam having a modulated frequency according to a different PN code, and measure an ultrasonic transit time; and a controller which collects the ultrasonic transit times measured by the plurality of set units, and calculates average flow velocity.
The set unit includes: a plurality of ultrasonic transducers which are disposed to be faced to each other at a different height at a upper stream and a lower stream of a fluid passage; a transmitting part for generating an ultrasonic signal having a modulated frequency according to a predetermined PN code; a transducer switching part for alternately applying the ultrasonic signal from the transmitting part to a pair of ultrasonic transducers, and switching a connection state of the pair of ultrasonic transducers for a transmitting or receiving operation; a receiving part for demodulating an output signal of the transducer switching part, and capturing a moment at which its own signal is identical with a signal of the transmitting part; a signal synchronization part for locking a synchronization of a receiving signal when the receiving part captures a signal; and a time interval measuring part for measuring an ultrasonic transit time under the condition that a synchronization of a receiving signal is locked in the signal synchronization part.